P0308 on 2005-2011 Cadillac STS: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2005-2011 Cadillac STS

Unlike older Northstar engines that used complex and expensive ignition 'cassettes', the 2005-2011 STS features a modern Coil-On-Plug (COP) system. This is good news for DIY repair, as it allows for the diagnosis and replacement of a single, relatively inexpensive coil. A unique quirk of this platform is a fuse labeled 'EVEN COILS' in the underhood fuse box; if this fuse blows, it cuts power to the entire driver's side bank of cylinders (2, 4, 6, and 8), which can mimic a much larger problem but is a very simple fix. 🎬 Watch: How to check the fuse when only 4 cylinders work. The LH2 Northstar in this vehicle is also considered the most refined version of the engine, with many of the earlier, notorious head gasket issues largely resolved.

Symptoms You May Notice

• Rough or vibrating idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Flashing Check Engine Light (indicates a severe misfire)
• Steady Check Engine Light
• Reduced fuel economy

Most Likely Causes

1. Faulty Ignition Coil on Cylinder 8 🔴 High Probability Coil-on-plug systems are a common failure point on many modern vehicles due to constant exposure to high heat and vibration. Cylinder 8's position at the back of the engine bay can sometimes lead to higher heat soak, accelerating wear.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6 on the same bank). Clear the codes and drive the vehicle. If the code changes to P0306, the ignition coil is faulty. This is the standard first diagnostic step after checking fuses.
   Typical fix: Replace the faulty ignition coil. It is recommended to use a quality OEM or aftermarket brand like ACDelco or Delphi. Given the labor to access the rear cylinders, some owners choose to replace all four coils on that bank at the same time.
   Est. part cost: $35-$80
2. Worn or Fouled Spark Plug 🔴 High Probability Spark plugs are a regular wear item. Oil or fuel fouling can also cause a misfire. The OEM-recommended service interval is up to 100,000 miles, but plugs can fail sooner, especially with city driving or if other issues like a minor valve cover leak are present.
   How to confirm: After confirming the coil is good, inspect the spark plug from cylinder 8. Look for heavy carbon buildup, oil fouling, a worn electrode, or a cracked porcelain insulator. You can also swap the plug with another cylinder to see if the misfire follows.
   Typical fix: Replace the spark plug for cylinder 8. Best practice is to replace all eight plugs at once 🎬 See this walkthrough on how to replace your spark plugs. if they are near the end of their service life. Use the correct OEM-spec plugs, such as ACDelco Iridium or Platinum plugs (Part No. 41-987).
   Est. part cost: $8-$20 per plug
3. Blown 'EVEN COILS' Fuse 🟡 Medium Probability A short in the wiring for any of the driver's side coils (Bank 2) can cause this fuse to blow, disabling all four cylinders on that bank (2, 4, 6, 8). This is a known quirk of the platform.
   How to confirm: Locate the fuse box in the engine bay (passenger side) and inspect the 15A fuse labeled 'EVEN COILS'. If it is blown, the engine will run very rough and likely set multiple codes (P0302, P0304, P0306, P0308).
   Typical fix: Replace the 15A fuse. If the fuse blows again immediately, there is a short circuit in the ignition coil wiring harness for that bank that must be diagnosed and repaired. This could be from a wire chafing against the engine block or a failed coil causing a direct short.
   Est. part cost: $1-$5
4. Clogged or Faulty Fuel Injector ⚪ Low Probability Fuel injectors can become clogged with deposits over time, leading to a lean misfire, or they can fail electrically. While not as common as ignition issues, it's the next logical step in diagnosis.
   How to confirm: Listen to the injector with a mechanic's stethoscope to hear if it's clicking. A more definitive test is to swap the injector with another cylinder, but this is more labor-intensive. A professional can perform an injector balance test with a GM Tech2 or equivalent scan tool.
   Typical fix: Replace the faulty fuel injector. It's also a good idea to replace the O-rings and seals.
   Est. part cost: $50-$120

Rare But Worth Checking

• Low Cylinder Compression: While rare for a single cylinder, a forum user reported a persistent P0308 was ultimately caused by a weak valve spring after all ignition and fuel parts were replaced. Other causes could be worn piston rings or a valve seat issue. A compression test followed by a cylinder leak-down test is required to diagnose this.
• Damaged Coil or Injector Wiring: The wiring harness leading to the coil or injector for cylinder 8 can become frayed, melted, or corroded, causing an intermittent connection. TSB #PIP4924D points to wiring harness chafing as a known issue for injector circuits on some GM vehicles. Also, check engine ground G102, located at the left rear of the engine, as it is the grounding point for all eight ignition coils.
• Intake Manifold Gasket Leak: A vacuum leak from a failed intake manifold gasket near the runner for cylinder 8 can cause a lean condition and trigger a misfire on just that cylinder. This may also be accompanied by a P0174 (System Too Lean Bank 2) code.
🎬 Watch: Troubleshooting rough idle and lean codes on your Cadillac.

Diagnosis Steps

1. Read the code with an OBD-II scanner and check for any other codes, such as P0300 or P0174.
2. CRITICAL FIRST STEP: Open the underhood fuse box (passenger side) and inspect the 15A 'EVEN COILS' fuse. If it's blown, replace it and see if the issue is resolved.
3. If the fuse is good, locate cylinder 8 (rearmost on the driver's side).
4. Swap the ignition coil from cylinder 8 with the coil from cylinder 6 (the next one forward on the same side).
5. Clear the codes with your scanner and test drive the vehicle until the check engine light returns.
6. If the new code is P0306, the ignition coil is the problem. Replace it.
7. If the code is still P0308, the coil is not the issue. Move the spark plug from cylinder 8 to cylinder 6.
8. Clear codes and test drive again. If the code changes to P0306, the spark plug is faulty. Replace it (and preferably all others if they are old).
9. If the code remains P0308, the problem is likely related to the fuel injector, wiring, or a mechanical issue with the cylinder.
10. Inspect the wiring and connector for the ignition coil and fuel injector on cylinder 8 for any damage or corrosion. Check the G102 engine ground at the left rear of the engine.
11. Listen to the fuel injector with a stethoscope for a consistent clicking sound. If it's silent or irregular, it may be faulty.
12. If no obvious issue is found, a professional diagnosis involving a fuel injector balance test, cylinder compression test, or cylinder leak-down test is recommended.

Parts You'll Likely Need

• Ignition Coil (OEM #12585765) — This is the most common failure part for a single-cylinder misfire on this engine. Heat and vibration cause the internal windings to fail over time.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $60-$90
  Aftermarket price range: $35-$60
• Spark Plug (OEM #41-987) — A worn or fouled spark plug is the second most common cause. It's a standard maintenance item that can fail and cause a misfire. The OEM part is an ACDelco Double Platinum plug.
  Trusted brands: ACDelco, NGK, Denso
  OEM price range: $15-$25
  Aftermarket price range: $8-$15
• Fuse (OEM #N/A) — The 15A 'EVEN COILS' fuse can blow due to a wiring short, disabling four cylinders. It's a cheap and critical diagnostic check.
  Trusted brands: Bussmann, Littlefuse
  OEM price range: $1-$5
  Aftermarket price range: $1-$5

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or other cylinders are also affected (e.g., by a vacuum leak or the blown 'EVEN COILS' fuse), a P0300 (Random/Multiple Cylinder Misfire) code may appear.
• P0302, P0304, P0306 — If the 'EVEN COILS' fuse is blown, you will likely have misfires on all cylinders on that bank, not just cylinder 8.
• P0174 — A vacuum leak on Bank 2 (the driver's side, where cylinder 8 is located) can cause a lean condition and set a P0174 code along with the misfire.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: A general diagnostic aid bulletin for technicians encountering misfire codes P0300-P0308, indicating that GM requires specific data collection (using tools like a PICO scope for relative compression tests) for complex cases before escalating to the Technical Assistance Center (TAC).
• PIP4924D: While not specific to P0308, this TSB addresses potential chafing of the fuel injector wiring harness on various GM vehicles, which could be a root cause if a wiring issue is suspected.

Platform-Specific Known Issues

• Corroded Engine Ground G102: The main ground point for all eight ignition coils is G102, located at the left rear of the engine. A loose or corroded connection here can cause weak, intermittent spark on any cylinder, including cylinder 8, leading to a misfire that is difficult to diagnose as parts-swapping may not reveal the root cause.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 42-50 PSI. Failure: Pressure below 42 PSI may indicate a weak fuel pump or a leak in the system.
• Fuel Pressure (At Idle) — expected: 35-45 PSI. Failure: Pressure dropping significantly below 35 PSI at idle could point to a failing fuel pressure regulator or pump.
• Mode $06 Misfire Counts (Cylinder 8) — expected: Near zero counts during current and last 10 driving cycles.. Failure: A high or steadily increasing count for Monitor ID $A9 (Cylinder 8) indicates an intermittent misfire that hasn't yet met the threshold to set a P0308 code.

Hidden / Shadow Codes Worth Checking

• Mode $06, Monitor ID $A9: This is not a DTC, but a specific monitor ID within the OBD-II system's Mode $06 data. It tracks the raw misfire count for cylinder 8. Test ID $0C shows counts for the current/last drive cycle, while $0B shows an average over the last 10 cycles. (see via An advanced OBD-II scanner or a professional tool like the GM Tech2 that has the capability to display Mode $06 data.)

Scan Tool Commands That Help

• GM Tech2 / GDS2: Cylinder Power Balance Test — This function is used when a misfire is felt but the specific cylinder is uncertain, or to confirm the severity of the P0308 misfire. The tool sequentially disables the fuel injector for each cylinder and measures the corresponding drop in engine RPM. If disabling cylinder 8 results in little or no RPM drop, it confirms that cylinder is not contributing power.
• GM Tech2 / GDS2: Fuel Injector Balance Test — After ruling out ignition components, this test can identify a faulty or clogged fuel injector. The tool commands each injector to fire for a specific duration while monitoring fuel pressure drop. An injector that causes a significantly different pressure drop compared to the others is likely faulty.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine block.. This is the primary ground connection for all eight ignition coils. A loose or corroded G102 can cause a weak or intermittent spark on any cylinder, including cylinder 8, and can be easily missed during diagnosis.
• G107 — Located on the rear side of the right cylinder head (Bank 1).. While G102 is the main coil ground, G107 is a critical engine ground on the opposite bank. Poor grounding anywhere on the engine can create electrical noise and potential issues for sensors and ignition components. It's worth checking all major engine grounds when diagnosing an electrical-type misfire.
• ECM (Engine Control Module) — For the 4.6L V8, the ECM is located below the driver's side headlight assembly.. This location is important for checking the main harness connector for corrosion or damage if a wiring issue between the ECM and the cylinder 8 coil or injector is suspected.
• S135 — A splice point located in the ignition coil harness, approximately 31 cm (12 inches) from the ignition coil 8 connector.. Factory splices can sometimes corrode or fail. If you suspect a wiring issue specific to the coil 8 circuit, this splice is a potential point of failure to inspect.

Real Owner Repair Stories

• CadillacForums user (2006 Cadillac STS 4.6L Northstar V8) — Persistent P0308 misfire code and rough running engine.
  ❌ Tried (didn't work) Replaced ignition coil, Replaced spark plug, Checked fuel injector and wiring
  ✅ What actually fixed it The final cause was a weak valve spring on cylinder 8. The issue was resolved by replacing the valve springs for that cylinder, which restored proper compression and valve seating.

"I Checked Everything" — The Actual Cause

• In cases where a smoke test for vacuum leaks comes back clean and swapping ignition/fuel parts doesn't move the misfire, the root cause is often mechanical. Technicians have found that a broken or weak valve spring can cause a persistent single-cylinder misfire that won't be detected by a smoke test. A compression test followed by a leak-down test is necessary to uncover this type of mechanical failure.

OEM Part Supersession History

• 12585765 → 89017778 (and various aftermarket equivalents) — The original part number for the ignition coil has been discontinued by GM.
  Heads up: While 89017778 is a potential GM replacement, direct interchange is not always guaranteed. It is critical to verify fitment using the vehicle's VIN. Quality aftermarket brands like Delphi or NGK are often used as reliable replacements.

Model Year Variations Within This Range

• 2008-2011: The STS received a facelift for the 2008 model year which included a new front grille and revised interior. While the base engine was upgraded to a direct-injected 3.6L V6, the 4.6L Northstar V8 powertrain remained largely unchanged through its final year in 2010. The diagnosis for a P0308 code is consistent across the entire 2005-2010 V8 model range.

Diagnostic Flowchart

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0300 — Random/Multiple Cylinder Misfire Detected. Often appears alongside P0308 if the misfire is severe enough to affect overall engine operation, or as noted in TSB PIP5628G.
• P0174 — System Too Lean (Bank 2). Can be triggered alongside P0308 because a misfiring cylinder pumps unburned oxygen into the exhaust, tricking the O2 sensor into reading a lean condition on the driver's side bank.
• P0302 — Cylinder 2 Misfire Detected. Frequently appears with P0308 if the 15A 'EVEN COILS' fuse blows, disabling the entire driver's side bank.
• P0304 — Cylinder 4 Misfire Detected. Frequently appears with P0308 if the 15A 'EVEN COILS' fuse blows, disabling the entire driver's side bank.
• P0306 — Cylinder 6 Misfire Detected. Frequently appears with P0308 if the 15A 'EVEN COILS' fuse blows. It is also the code you will see if you swap a faulty coil or plug from cylinder 8 to cylinder 6 during diagnosis.
• P0019 — Crankshaft Position - Camshaft Position Correlation (Bank 2 Sensor B). As noted by owners of the shared LH2 engine, this code can accompany P0308 when the root cause is a deeper engine timing deviation rather than a simple ignition failure.

Frequently Asked Questions